The present invention relates to a liquid crystal panel comprising a pair of base plates, at least one of which is a plastic plate, and a liquid crystal material interposed between the base plates.
Most liquid crystal panels practically used heretofore have a structure in which a pair of transparent base plates such as those of inorganic glass or plastic are respectively provided with a transparent electrode, e.g., of tin oxide, indium oxide or a composite of these and subjected to a homogeneous or horizontal orientation treatment, and a liquid crystal is disposed between the base plates, whereby a voltage can be applied between the electrodes. As methods, for the homogeneous or horizontal orientation treatment, the following methods (a) to (d) have been conducted heretofore.
(a) Rubbing of a base plate with, e.g., a cotton cloth in one direction.
(b) Oblique or tilt vapor deposition of SiO or SiO.sub.2.
(c) Rubbing of a film of an organosilane, a linear polyamide, a polyimide, etc., formed on a base plate.
(d) Irradiation of a high energy beam such as electron beam or plasma discharge.
The above methods are effective for the homogeneous orientation treatment of inorganic glass plates but have not been fully effective with respect to plastic substrates.
Thus, in the method (a), the resultant orientation performance does not last for a long term and cannot resist aging at 45.degree. C. or above. In the method (b), the temperature at which the base plate is heated to ensure the intimate adhesion thereto of deposited SiO or SiO.sub.2 film exceeds the heat deformation temperature of a plastic plate (below about 150.degree. C.) and causes deformation of the base plate. On the contrary, if the vapor deposition is conducted while the base plate is heated to a lower temperature, the deposited film has a poor adhesion to the plastic base plate and has a low resistance to aging. In the method (c), an orientation controlling aid such as an organosilane (a trialkoxymonoalkylsiloxane, ordinarily called a silane coupling agent, is generally used and, especially, 3-aminopropyltriethoxysilane is frequently used) has a poor resistivity to aging. On the other hand, when a polyimide film which is most widely used is intended to be formed, the plastic base plate cannot resist the temperature (250.degree. C. or above) required for polymerizing a polyamide acid (or a polyamide-imide) which is a precursor of a polyimide. In the method (d), due to irradiation with a high energy beam, the plastic plate or transparent electrode deteriorates. Moreover, this method is not adapted for mass production, and an advantage from the viewpoint of production resulting from the use of a plastic base plate is lost.
Further, a biaxially stretched polyester film having a thickness of the order of 100 .mu.m has been used as a plastic film substrate for TN-type liquid crystal devices in view of various properties such as film properties (heat-deformation resistance, chemical resistance, gas-barrier property), appearance (transparency, smoothness), cost, electrical resistance (100-1 k.OMEGA./cm.sup.2) and adhesion to the substrate of a transparent electrode (formed, e.g., by vapor deposition or low-temperature sputtering of indium oxide on a surface-treated film), and little change in performance with the elapse of time. However, a biaxially stretched polyester film has optical anisotropy as shown in FIG. 4. In FIG. 4, reference numeral 21 denotes a biaxially stretched polyester film, 22 directions of film stretching, and 23 a direction of an optical axis (direction of optical rotation). As the biaxially stretched polyester film has an optical anisotropy as shown in the figure, it is accompanied with a difficulty in selection and combination of optical axes of the films and a problem such as occurrence of interference color depending on visual field angle. In order to solve these problems, a monoaxially stretched polyester film has been used recently. However, a monoaxially stretched polyester film has a larger heat shrinkage in the MD direction (machine direction) compared with a biaxially stretched polyester film. Therefore, a uniform gap of the order of 10 .mu.m required for a liquid crystal panel cannot be obtained by mere adjustment of films with respect to the optical axes as has been conventionally carried out. Further, a large displacement between the pair of electrodes has occurred due to heat shrinkage, whereby degradation of display qualities has occurred or poor continuity at contacts with the pair of electrodes has occurred.
Table 1 below shows comparison of heat shrinkages between a biaxially stretched polyester film and a monoaxially stretched polyester film after they were heated for 1 hour in an oven of 150.degree. C.
TABLE 1 ______________________________________ Film Biaxially stretched Monoaxially stretched Direction polyester film polyester film ______________________________________ TD* -0.23% 0.05% MD* 0.56% 1.6% ______________________________________ *TD: Transverse direction MD: Machine direction